1. Field of the Invention
The present invention relates to a power factor correction controller, and more particularly to a power factor correction controller capable of simultaneously controlling several power converters to improve the overall conversion efficiency and conversion power of a power factor corrector.
2. Description of Related Art
Since the present household or industrial electric appliances have a voltage difference with the input power supply terminal (such as a public utility electricity terminal) or even have an input current with a high-peak factor (harmonics), and thus the quality of electricity drops, and it is necessary to have a power factor corrector for the power factor correction and harmonic suppression. The main function of the power factor corrector is to compensate the phase difference of the current with respect to the voltage of the electric appliances and suppress the harmonic current produced by the electric appliances, so as to avoid adverse effects on the quality of electric power. In general, an electric company prefers simply connecting a resistor load to a power circuit to producing currents with a high-peak value, since the currents with a high-peak value may fuse a circuit breaker easily and cause a disorder to a voltage regulating circuit.
The power factor corrector is generally divided into a power stage and a control stage. Referring to FIG. 1 for the circuit diagram of a prior art power factor corrector, the electric structure of a power factor corrector 32 includes a rectifying circuit 30 for converting an input of public utility electricity AC into a DC power, and a load 34 is another electric circuit of an electric appliance. As to the power stage 322 of the power factor corrector 32, there are several common topological structures: a boost structure, a buck structure and a flyback structure. In these structures, the boost structure uses a single stage circuit to achieve a high power factor and a lower harmonic effect, and thus it is the most popular one used for a power factor corrector 32. As to the control stage 324, it is divided into two main types: a continuous conduction mode or a critical conduction mode depending on the operating mode of its power converter (power stage 322). If the inductor current of the controlled power converter is in a continuous conduction (except when the input voltage is zero, the inductor current will be greater than zero for each cycle) or in a critical conduction (the inductance will drop to zero for each cycle), and both use a control circuit 3241 to process the signals including a feedback output voltage, an input current and an input voltage, and determine the gate control signal of a power switch component Q at a drive stage 322, so that a high frequency switch is used to force the input current to follow the reference current signal determined by the voltage waveform of the public utility electricity, so as to achieve the power factor correction.
At present, most of the control stages of the continuous conduction mode power factor corrector use the UC3854 IC (or other equivalent ICs) for the control. Referring to FIG. 2 for the circuit diagram of a prior art power factor correction controller that employs the UC3854 IC as a controller, a control circuit 26 of the UC3854 includes three parts: a current feedback control stage 266, a voltage feedback control stage 264 and a feedforward control stage 262. Most critical conduction mode power factor correctors use the L6561 IC (or other equivalent ICs) for the control. Referring to FIG. 3 for the circuit diagram of a prior art power factor correction controller that employs the L6561 IC as a controller, a control circuit 46 of the L6561 IC includes four parts: a current reference signal feedback control 462, a voltage feedback control 464, a current feedback control 466 and a drain-source zero voltage detecting circuit 468. The operating method for both modes are prior arts and thud will not be described further here, and only their advantages and disadvantages will be given below.
The UC3854 IC controls the power converters to be operated in the continuous conduction mode, and thus having the advantages of a high conversion power and a low input current ripple. Since the power switch of the power converters is operated at a compulsory switching mode, therefore the UC3854 IC has the disadvantage of a low conversion efficiency. Further, this control includes three major parts: a feedforward feedback control, a voltage feedback control and a current feedback control. For a high conversion power, it requires other auxiliary circuits to flexibly switch the power switch of the main circuit in order to improve the conversion efficiency, and thus additional control of at least one set of auxiliary circuits is needed, and the control will become very complicated. The L6561 IC controls the converter to be operated at a critical conduction mode, and thus the power switch must have a flexible switching characteristic, and the power converter has high conversion efficiency. However, the input current ripple of the power factor corrector operated at the critical conduction mode is larger, and thus requiring a specification for larger components and a pre-filter to achieve the power factor correction, since its conversion power is usually not high (or below 100 W).
To overcome the foregoing shortcomings, the present invention provides a power factor correction controller, not only controlling the power factor corrector to achieve a high power factor effect, but also providing a high conversion efficiency and a high conversion power without producing a high input current ripple at the input terminal.